1. Field of the Invention
The present invention relates to a fixing device that heats and melts an unfixed toner and fixes the toner to a sheet, while passing the sheet carrying the toner image by nipping between a pair of heated rollers or a heating belt and a roller, and also to an image forming apparatus in which the fixing device is installed.
2. Description of the Related Art
In the image forming apparatuses of the aforementioned kind, a belt system in which a low thermal capacity can be set has recently attracted attention due to a demand for a short warm-up time and reduced energy consumption in the fixing device. An electromagnetic inductive heating system (IH) that enables rapid heating and high-efficiency heating has also attracted attention in recent years, and a large number of products in which the electromagnetic inductive heating is combined with the belt system to reduce energy consumption when fixing color images have been produced. When the belt system is combined with electromagnetic inductive heating, the electromagnetic inductive units are in most cases disposed outside the belt because such a configuration (so-called external inductive heating IH) ensures simple coil layout and cooling and enables direct heating of the belt.
A variety of techniques have been disclosed for preventing an excessive increase in temperature in the sheet non-passage area according to a width of sheet passing through the fixing device (sheet passage width). In particular, the below-described first prior art technique and second prior art technique are used as a size switching means in the external IH.
With the first prior art technique, a magnetic member is divided into a plurality of sections that are arranged side by side in the sheet passage width direction, and some of the magnetic members are withdrawn from or brought closer to an excitation coil according to the sheet size (sheet passage width). In this case, in the sheet non-passage area, the magnetic members are withdrawn from the excitation coil to decrease heat generation efficiency, and the amount of generated heat is reduced with respect to that of the area corresponding to the sheet of the minimum sheet passage width.
With the second prior art technique, a separate electrically conductive member is disposed outside the minimum sheet passage width inside a heat-generating roller, and the position of the electrically conductive member is switched from that inside to that outside the magnetic field range. With such a prior art technique, the electrically conductive member is initially positioned outside the magnetic field range, the heat-generating roller is electromagnetically inductively heated, and where the heat-generating roller rises to a temperature close to a Curie temperature, the electrically conductive member is moved into the magnetic field range, whereby the magnetic flux is prevented from leaking from the heat-generating roller outside the minimum sheet passage width and an excess increase in temperature is prevented.
However, with the size switching means based on the above-described first prior art technique and second prior art technique, the effect of inhibiting an excessive increase in temperature has to be further improved over the presently attained one in order to increase productivity. For example, in order to improve the effect of inhibiting an excessive increase in temperature over the presently attained one with the second prior art technique, the surface area of the electrically conductive member that conducts magnetic shielding may be increased over the presently used one.
However, where the increase in surface area of the electrically conductive member is too large, the electrically conductive member is difficult to withdraw completely from the magnetic field range, and even if the larger portion is withdrawn to the outside of the magnetic field range, the remaining portion still can affect the magnetic field. Therefore, from the standpoint of increasing the effect of inhibiting an excessive increase in temperature, the expansion of surface area of the electrically conductive member is limited.
When this problem is resolved, it is undesirable to arrange in a row a plurality of individual electrically conductive members that shield a magnetic field. This is because where a space is formed between the adjacent electrically conductive members, no magnetic shielding effect is demonstrated in this space, a magnetic field leakage channel can be formed, and the fixing device is increased in size.